1. Technical Field
This invention relates in general to inductively driven electrodeless gas discharge lamps and, in particular, to electromagnetic shielding of radio frequency interference emitted from the drive coils used to energize such lamps.
2. Description of the Related Art
Inductively driven electrodeless gas discharge lamps utilize a solenoidal coil driven with alternating current to produce a plasma discharge within the lamp envelope. Alternating current flow through the coil generates a time-varying magnetic field that impinges upon the ionizable gas fill within the lamp, causing it to produce the plasma discharge. The gas fill can be an inert or other rare earth gas, such as neon, which produces a visible discharge when excited by the magnetic field.
Often, these lamps are driven at radio frequencies resulting in strong magnetic and electric fields that radiate well beyond the lamp envelope. In many applications, these radiated fields can detrimentally effect the operation of nearby circuits and sensors. For example, when used in automotive applications, the drive coils used to energize these lamps could detrimentally influence such things as engine sensors and the vehicle's electronic compass. Accordingly, it is well known to shield the electromagnetic radiation emanating from the drive coil. Although electric and magnetic field shielding is most effectively accomplished with a grounded, electrically conductive, ferromagnetic enclosure, such an arrangement is not practical since it would also shield the light transmitted by the lamp, making the entire assembly useless for its intended purpose.
Consequently, shielding is typically accomplished using a conductive screen or wire mesh that extends over all or a portion of the lamp envelope. See, for example, U.S. Pat. No. 5,397,966 to Vrionis et al. One problem with the use of a wire mesh for shielding purposes is that it can cause a significant reduction in the light output from the lamp due to the large total area covered by the wire making up the mesh. Vrionis et al. also disclose a shield made up of a plurality of electrically conductive fingers that extend generally in the direction of the axis of the induction coil. This arrangement of conductive fingers helps reduce any detrimental effect that the shield has on the efficiency of the lamp operation; however, it is believed that it also results in a correspondingly reduced effectiveness of the magnetic shielding effect.